Scout was a towel boy, I mean, assistant manager, for the college baseball team. Then one day they were down a man, and sent him out to play. The instant he was given that bat, he just lost his head, or, everyone else lost THEIR heads.
Soldier went to a school in Russia and blew it up.
Pyro was a physics teacher who fell in love with a chemistry teacher, and then he went crazy after a freak napalm experiment.
Demoman never went to college, his parents died and he had no way of funding for college.
The Heavy went to a cooking class. How else do you think he makes such delicious sandviches? For goodness sake the sheer taste of them alone heals bullet wounds.
Engineer: covered
Sniper never went to college, he lived on the open road, but picked up things in his travels.
Medic was a mortitioner, and he quit because the bodies didn't squirm.
The Spy has gone to many schools impersonating a teacher, a student, a janitor, a chair, a pencil, and a projector that somehow worked.